New Materials and Architectures for Organic Photovoltaics

Abstract

There are growing demands for energy throughout the world. In order to meet the rising energy pressures of the future, renewable sources are required. One approach to resolve this problem is organic photovoltaics (OPVs), which offers a potential low-cost energy solution for the future. Before this technology is commercially feasible, improvements in efficiency, lifetimes, mechanical stability, and processing are required. This thesis presents an integrative approach to investigating the scalability, lifetime stability, and mechanical properties of OPVs. A robust spray coating method was developed for high conductivity poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate) (PEDOT:PSS) transparent electrodes. Conductivities of >1000 S cm-1 are achieved with sheet resistances of 24 ohm sq-1 and 74% transmittance, which are amongst the best-reported in the field. OPV devices fabricated with the high conductivity transparent electrodes yielded power conversion efficiencies (PCEs) of 3.2%. Mechanical bending and stretching tests demonstrated that the flexibility of these polymer layers were far superior to that of indium tin oxide (ITO). Collectively, our results illustrate a promising future for the scalable printing of low-cost PEDOT:PSS-based flexible transparent electrodes. A water-soluble cationic polythiophene derivative was combined with anionic PEDOT:PSS on ITO substrates via electrostatic layer-by-layer (eLbL) assembly. By varying the number of eLbL layers, the electrode's work function was precisely controlled from 4.6 to 3.8 eV. These polymeric coatings were used as cathodic interfacial modifiers for inverted-mode organic photovoltaics. The PCE of the photovoltaic device was dependent on the composition of the eLbL-assembled interface and permitted fabrication of devices with efficiencies of 5.6%. Notably, these devices demonstrated significant air stability, maintaining 97% of their original PCE after over 1000 h of storage in air. The optoelectronic and photophysical properties of four regioregular poly[3-(carboxyalkyl)thiophene-2,5-diyl] (P3CAT) with different carboxyalkyl chain lengths (propyl to hexyl) are reported. Each P3CAT is combined with functionalized C60 to form the photoactive bulk heterojunction layer for OPV devices. The extent of hydrogen bonding and polymer crystallinity in the films was determined, and the mechanical properties of films suggested that P3CATs were suitable for use in flexible devices. PCEs of up to 2.6% and 1.6% were obtained for devices on glass and plastic substrates, respectively.